If I have a person with a blue hat and I want everything else in the clip except the hat to be transparent, how do I do this?
this would be as a "invert" color key in Pr
You either apply an Invert effect after the key or use the keyed layer as an inverted track matte on a a duplicate of the original source. Pre-composing the key may be necessary to get correct results.
What key should I use, there is no color key in Ae?
The most efficient way to pull a procedural matte (color key) depends entirely on the shot. If the blue hat is fairly evenly lit you can use Keylight. You may need to do some serious color correction before you can get any keying or matte-creating software to do a good job.
Post a screenshot of the footage and I'll be glad to help you figure it out. If you can pull a good key in Premiere Pro and that's all you need, then there is no need at all for an AE comp to pull the key. Just render a Lossless With Alpha copy of the footage from Premiere Pro.
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I used linear color key, invert and alpha chanel.. This worked great. I think I will use a black hat so
tne color should be possible to get fairly even.
Be careful, black can be extremely difficult to use as a source for a matte. Any very bright hat that is a different color than any other item in the shot would be better than black. Even if the lighting is not perfect.
OK, If I use another color than black, can I change this color to black after so it will be black in the final clip?
Just use the matte to isolate the hat, add the Black And White color correction effect, and fine-tune for the final color grade.
Yes, that shold be the easy part.